A protester holds a placard showing an employer devouring a young employee at a rally in Paris. (Reuters)

Paris, March 18 (Reuters): Huge crowds of students, trade unionists and Left-wingers took to the streets across France today to put pressure on the conservative government to cancel a new law they fear will undermine job security for young workers.

In a festive mood under bright blue skies, tens of thousands turned out in Paris, Lyon and Rennes in the biggest of 160 planned demonstrations in a growing movement that has created a serious crisis for Prime Minister Dominique de Villepin.

Trade union crowd marshals and ranks of riot police kept discreet but watchful eyes on the crowds to avoid any repetition of the violence that marred a Paris student rally on Thursday.

The protesters demand that Villepin withdraw a new youth job contract, known as the CPE, which lets firms fire workers under 26 without explanation in their first two years on the job. He launched it to spur reluctant employers to take on new staff.

“I risk working for two years for nothing, just to be fired at any moment,” said Paris student Coralie Huvet, 20, who had “No to the CPE” written on her forehead. Pointing to painted-on tears, she added: “That’s depressing, that’s why I’m crying.”

Organisers, who decry the CPE as a “Kleenex contract” that lets young workers be “thrown away like a paper tissue,” said they hoped to have up to 1.5 million people out marching in the third national protest in six weeks.

France’s main trade union leaders led the Paris march, followed by dancing and singing high school and university students and then ranks of workers.

Opposition Socialist and Communist politicians also joined the protest, only the third time in almost four decades ' after 1968 and 1994 ' that students and workers marched together. Many parents accompanied their children in the march, where banners declared “No to throw-away youths” and “Tired Of Being Squeezed Lemons.”

Union leaders threatened to keep up the pressure on the government with further action next week.

“If they don’t listen to us we are going to have to think about moving to a general strike across the whole country,” said Bernard Thibault, head of the pro-Communist CGT union.